A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. In an embodiment, the liquid is distilled water, although another liquid can be used. An embodiment of the present invention will be described with reference to liquid. However, another fluid may be suitable, particularly a wetting fluid, an incompressible fluid and/or a fluid with higher refractive index than air, desirably a higher refractive index than water. Fluids excluding gases are particularly desirable. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. (The effect of the liquid may also be regarded as increasing the effective numerical aperture (NA) of the system and increasing the depth of focus.) Other immersion liquids have been proposed, including water with solid particles (e.g. quartz) suspended therein, or a liquid with a nano-particle suspension (e.g. particles with a maximum dimension of up to 10 nm). The suspended particles may or may not have a similar or the same refractive index as the liquid in which they are suspended. Other liquids which may be suitable include a hydrocarbon, such as an aromatic, a fluorohydrocarbon, and/or an aqueous solution.
Submersing the substrate or substrate and substrate table in a bath of liquid (see, for example, U.S. Pat. No. 4,509,852) means that there is a large body of liquid that must be accelerated during a scanning exposure. This requires additional or more powerful motors and turbulence in the liquid may lead to undesirable and unpredictable effects.
In an immersion apparatus, immersion fluid is handled by a fluid handling system, structure or apparatus. In an embodiment, the fluid handling system may supply immersion fluid and may therefore be a fluid supply system. In an embodiment, the fluid handling system may at least partly confine immersion fluid and thereby be a fluid confinement system. In an embodiment, the fluid handling system may provide a barrier to immersion fluid and thereby be a barrier member, such as a fluid confinement structure. In an embodiment, the fluid handling system may create or use a flow of gas, for example to help in controlling the flow and/or the position of the immersion fluid. The flow of gas may form a seal to confine the immersion fluid and so the fluid handling structure may be referred to as a seal member; such a seal member may be a fluid confinement structure. In an embodiment, immersion liquid is used as the immersion fluid. In that case, the fluid handling system may be a liquid handling system. In reference to the aforementioned description, reference in this paragraph to a feature defined with respect to fluid may be understood to include a feature defined with respect to liquid.
One of the arrangements proposed is for a liquid supply system to provide liquid on only a localized area of the substrate and in between the final element of the projection system and the substrate using a liquid confinement system (the substrate generally has a larger surface area than the final element of the projection system). One way that has been proposed to arrange for this is disclosed in PCT patent application publication WO 99/49504. As illustrated in FIGS. 2 and 3, liquid is supplied by at least one inlet IN onto the substrate, preferably along the direction of movement of the substrate relative to the final element, and is removed by at least one outlet OUT after having passed under the projection system. That is, as the substrate is scanned beneath the element in a −X direction, liquid is supplied at the +X side of the element and taken up at the −X side. FIG. 2 shows the arrangement schematically in which liquid is supplied via inlet IN and is taken up on the other side of the element by outlet OUT which is connected to a low pressure source. In the illustration of FIG. 2 the liquid is supplied along the direction of movement of the substrate relative to the final element, though this does not need to be the case. Various orientations and numbers of inlets and outlets positioned around the final element are possible. One example is illustrated in FIG. 3 in which four sets of an inlet with an outlet on either side are provided in a regular pattern around the final element.
A further immersion lithography solution with a localized liquid supply system is shown in FIG. 4. Liquid is supplied by two groove inlets IN on either side of the projection system PL and is removed by a plurality of discrete outlets OUT arranged radially outwardly of the inlets IN. The inlets IN and OUT can be arranged in a plate in which is defined a hole, which may be in its center, and through which the projection beam is projected. Liquid is supplied by one groove inlet IN on one side of the projection system PL and removed by a plurality of discrete outlets OUT on the other side of the projection system PL, causing a flow of a thin film of liquid between the projection system PL and the substrate W. The choice of which combination of inlet IN and outlets OUT to use can depend on the direction of movement of the substrate W (the other combination of inlet IN and outlets OUT being inactive).
Another arrangement which has been proposed is to provide the liquid supply system 12 with a barrier member which extends along at least a part of a boundary of the space between the final element of the projection system and the substrate table, as depicted in FIG. 5. The barrier member is substantially stationary relative to the projection system in the XY plane though there may be some relative movement in the Z direction (in the direction of the optical axis). A seal is formed between the barrier member and the surface of the substrate. In an embodiment, the seal is a contactless seal such as a gas seal 16. Such a system with a gas seal is disclosed in U.S. Patent Application Publication No. 2004/0207824 as well as European Patent Application Publication No. EP 1 429 188 and U.S. Patent Application Publication No. 2004/0211920.
In European Patent Application Publication No, EP 1 420 300 and United States Patent Application No. 2004/0136494, the idea of a twin or dual stage immersion lithography apparatus is disclosed. Such an apparatus is provided with two tables or stages for supporting the substrate. Leveling measurements are carried out with a table at a first position, without immersion liquid, and exposure is carried out with a table at a second position, where immersion liquid is present. Alternatively, the apparatus may have only one table movable between exposure and measurement positions.
PCT patent application publication WO 2005/064405 discloses an “all wet” arrangement in which the immersion liquid is unconfined. In such a system, the whole top surface of the substrate is covered in liquid. This may be advantageous because then the whole top surface of the substrate is exposed to the substantially same conditions. This has an advantage for temperature control and processing of the substrate. In WO 2005/064405, a liquid supply system provides liquid to the gap between the final element of the projection system and the substrate. That liquid is allowed to leak over the remainder of the substrate. A barrier at the edge of a substrate table prevents the liquid from escaping so that it can be removed from the top surface of the substrate table in a controlled way. Although such a system improves temperature control and processing of the substrate, evaporation of the immersion liquid may still occur. One way of helping to alleviate that problem is described in United States patent application publication no. 2006/0119809. A member is provided which covers the substrate in all positions and which is arranged to have immersion liquid extending between it and the top surface of the substrate and/or substrate table which holds the substrate.
A problem in immersion lithography is the occurrence of a bubble in the immersion liquid. If the path of the patterned beam passes through a region or volume of immersion liquid that contains a bubble, this may deleteriously affect the quality of the pattern image projected onto the substrate. For example, the imaged pattern may contain consequential defects. The presence of a bubble in the immersion liquid, particularly in the immersion space, is to be avoided.
There may be a recess formed in a surface opposing the liquid confinement structure. For example, a recess may exist in a substrate table or between a substrate and the substrate table, or even between the substrate table and a sensor, swap bridge, closing disc or measurement stage that is positioned in or next to the substrate table. The recess may be a groove which may function as a gutter. The recess may be configured to remove excess immersion liquid from the surface opposing the liquid confinement structure. The recess may trap gas when the substrate table moves relative to the liquid confinement structure or radiation system. The recess may cause liquid to escape from the immersion space (into the recess), resulting in an overall drop in liquid level in the immersion space. A drop in immersion liquid level could be a source of a bubble inclusion in the immersion space. Fluid behavior in the region of the recess may cause a bubble and/or a droplet to form. The droplet may cause a bubble to form in the immersion space. The droplet may apply a heat load onto a surface onto which it lands, for example the surface of a substrate or substrate table. The bubble or droplet may therefore cause deterioration, for example, in lithographic imaging performance.